A simplified view of protein translocation across the ER membrane, as originally proposed. When the signal peptide emerges from the ribosome, it directs the ribosome to a receptor protein on the ER membrane. As it is synthesized, the polypeptide is postulated to be translocated across the ER membrane through a protein pore associated with the receptor. The signal peptide is clipped off during translation by a signal peptidase, and the mature protein is released into the lumen of the ER immediately after being synthesized. We now know that the hypothesis is correct in outline but that additional components besides those shown in this figure are required. The signal peptidase, for example, is a complex of five different membrane-bound polypeptide chains, with one complex apparently associated with every translocation pore.